Such transformer is known from Japanese Patent Publication No. 2002-217050 and No, 2002-93635 to include an inner winding placed directly around a bar-shaped ferrite core, an outer winding wound over the inner winding, and a dielectric shield surrounding the windings. The dielectric shield is formed by an injection molding and is made of unsaturated polyester which is a thermosetting dielectric resin and is mixed with a filler enhancing heat-resistance as well as impact-strength.
In the transformer, the outer winding is wound along substantially the full length thereof in a closely relation to the inner winging. In this condition, there appears a substantially closed space within one turn of the outer winding around the inner winding. The substantially closed space is a space into which the dielectric molding resin including the filler is difficult to flow during the injection molding, and therefore remains as a void after forming the dielectric shield. As a result of that the void free from the resin remains, the transformer suffers from a lowered dielectric strength and therefore fails to give an intended voltage difference between the outer and inner windings over a long period of use. For example, in case 800 volts are applied to the outer winding of 5 turns with the inner winding of 200 turns, there is developed a 10 to 20 kV across the inner windings. In the presence of the void between the outer and inner windings, the voltage of 10 to 20 kV is likely to develop a corona discharge responsible for an aged deterioration of the dielectric strength which eventually causes a discharge between the outer and inner windings, thereby failing to give the intended high voltage from the inner winding.